Biomechanical Principles

• Biomechanics

  the study of human movement & the effect of force & motion on sports performance
• What Does Biomechanics Enable Performers & Coaches to do

  • analyse performance
  • maximise movement efficiency & sporting technique
  • reduce or prevent injuries
  • design & choose the correct equipment to satisfy demands of the activity
• Force

  push/pull that alters the state of motion of a body
• Inertia

  the resistance of a body to change its state of motion, whether at rest or while moving
• Velocity

  the rate of change in displacement, how fast a body moves in a specific direction
• Momentum

  the quantity of motion possessed by a moving body, the product of its mass & velocity
• Acceleration

  the rate of change in velocity
• Newtons 3 Laws of Motion

  1. law of inertia
  2. law of acceleration
  3. law of reaction
• Law of Inertia

  a body continues in a state of rest or uniform velocity unless acted upon by an external or unbalanced force

  • an object wont change its speed until something makes it (friction)
  • the greater the mass, the greater the inertia

  e.g a ball at rest will not move until kicked by a footballer
• Law of Acceleration

  a bodys rate of change in momentum is proportional to the size of the force applied & acts in the same direction as the force applied

  • how quickly an object speeds up is due to how big the force is & in what direction its applies
  • greater force = greater acceleration
  • greater mass of object = greater force required

  e.g the greater the force applied to a sprinter, the greater the rate of change in momentum & therefore acceleration away from the blocks
• Law of Reaction

  for every action force applied to a body, there is an equal & opposite reaction force

  • if you push an object, the object pushes back the same amount
  • reaction force is commonly used as feedback when performing a skill

  e.g when a sprinter applies a down & backwards action force into the blocks, the blocks provide an equal & opposite reaction force to the sprinter, allowing them to drive out the blocks
• Velocity

  the rate of change in displacement

  velocity = displacement / time taken
• Momentum

  the quantity of motion possessed by a moving body

  momentum = mass X velocity
• Acceleration

  the rate of change in velocity

  acceleration = (final velocity - initial velocity) / time taken
• Force 


  a push/pull that alters the state of motion of a body

  force = mass X acceleration
• Forces that Act on a Body

  • air resistance
  • friction
  • water resistance
  • weight
• The 5 Effects of Force

  1. create motion
  2. accelerate a body
  3. decelerate a body
  4. change the direction of a body
  5. change the shape of a body
• Net Force

  • the sum of all forces acting on a body (resultant force)
  • different forces of a body are added together
  • this determines whether the forces are balanced or unbalanced
• Balanced Forces

  • net force = 0
  • multiple opposing forces on an object are equal

  e.g when a rugby scrum doesn't move due to equal force from both teams
• Unbalanced Forces

  • net force is present
  • multiple opposing forces on an object are unequal
  • object will move in direction of larger force

  e.g if one team applies a greater force in a rugby scrum, the scrum will move forwards
• Vertical Forces

  • weight
  • reaction
• Weight

  • the gravitational pull that the earth exerts on a body
  • weight force is always present & acts downwards from the boys CoM
• Reaction

  • the equal & opposite force exerted by a body on response to the action force placed upon it
  • result of Newtons third law of motion, & is always present when 2 bodies are in contact
  • if the force exerted is larger than the reaction force, it creates an upwards movement
• Horizontal Forces

  • friction
  • air resistance
• Friction

  • the force that opposes the motion of 2 surfaces in contact
  • then grip is desired, greater friction is needed

  Factors Affecting Friction:

  roughness of the ground surface

  roughness of the contact surface

  temperature

  size of normal reaction
• Air Resistance

  • the force that opposes the motion through the air
  • its a form of fluid friction

  Factors Affecting Air Resistance:

  frontal cross-sectional area

  velocity

  shape

  smoothness of surface
• Free Body Diagrams

  a clearly labelled sketch showing all of the force acting on a body at a particular instant in time

  they clearly show the origin of the force, the size of the force, & the direction the force is acting in
• Free Body Diagrams - Vertical Forces

  • weight - force of arrow should go from CoM extending vertically downwards
  • reaction - force of arrow should go from point of contact extending vertically upwards

  Relationship Between Forces:

  if W=R, net force is zero so body will remain at rest

  if R>W, net force is positive & therefore acceleration in an upwards direction will occur
• Free Body Diagrams - Horizontal Forces

  • friction - arrow should go from point of contact extending horizontally in the same direction as motion
  • air resistance - arrow should go from CoM extending horizontally against the direction of motion

  Relationship Between Forces:

  if F=AR, net force is zero therefore the body will continue to move at constant velocity

  if F>AR, net force is positive so there will be acceleration in a forwards direction

  if AR>F, net force is negative so there will be horizontal deceleration